At present, radio communication systems such as portable telephone systems and wireless LANs (Local Area Networks) are in widespread use. And in the field of radio communication, next-generation radio communication technologies are being continuously and actively discussed as means of further improving communication speeds and capacities.
The OFDM (Orthogonal Frequency Division Multiplexing) method is one communication method among such radio communication technologies. FIG. 19A illustrates an example of the configuration of a transmission apparatus 150 using the OFDM method. In the OFDM method of communication, for example a frequency band is divided into a plurality of frequency bands (or subcarriers), and information data or similar is mapped to orthogonal frequency bands and transmitted. Hence compared with other methods, OFDM has the characteristic of superior efficiency of frequency utilization. However, in the OFDM method, random information data on the frequency axis is subjected to IFFT (Inverse Fast Fourier Transform) processing to generate time-axis signal. For this reason, compared with other communication methods, OFDM has the characteristic of a high PAPR (Peak to Averaged Power Ratio).
The SC-OFDM (Single Carrier OFDM) is another communication method which is attracting attention. FIG. 19B illustrates an example of the configuration of a transmission apparatus 160 using the SC-OFDM method. The SC-OFDM method is a communication method in which for example a frequency band is divided, and different frequency bands are used for transmission between a plurality of terminals. The SC-OFDM method has smaller amplitude fluctuation of time-axis signal compared with the OFDM method, and the PAPR can be lowered. Hence a transmission apparatus 160 using the SC-OFDM method can reduce power consumption compared with a transmission apparatus 150 using the OFDM method.
Whether OFDM or SC-OFDM is used, such transmission apparatuses 150 and 160 transmit transmission signal, including preamble signal and a plurality of data signal, as radio frames to a reception apparatus. Among these, preamble signal include a bit pattern which is known by both the transmission apparatus 150, 160 and the reception apparatus, and for example is used in reception synchronization in the reception apparatus. For example, the reception apparatus detects the correlation peak power in the preamble section, and establishes reception synchronization with reference to the time of detection of the correlation peak power. FIG. 20A illustrates an example of correlation power in a preamble section, measured in a reception apparatus. In FIG. 20A, the horizontal axis represents time and the vertical axis represents correlation power. In this example, there are three symbols' worth of preamble signal in one radio frame; consequently, in FIG. 20A there are three portions which are correlation peak powers. The reception apparatus for example extracts the times at which the difference between the correlation peak power and the second large correlation power is equal to or above a threshold value, or is equal to the correlation peak power, and establishes reception synchronization with this time as reference.
The following are two such preamble-related techniques. Regarding preamble patterns, by transmitting two sub-patterns, which are “P2” and a pattern “−P2” with phase inverted relative to “P2”, when using a single-carrier method to realize MIMO (Multiple-Input and Multiple-Output) transmission, increases in the preamble interval are suppressed.
Further, there are apparatuses in similar in which, by using cross-correlation characteristics of preamble codes and autocorrelation determination to identify a preamble code, and estimating integer carrier frequency offsets, preamble codes can be detected quickly and correctly even in an environment in which there are carrier frequency offsets.    Japanese Laid-open Patent Publication No. 2009-135866    Japanese Laid-open Patent Publication No. 2008-236744
A radio signal transmitted from transmission apparatuses 150, 160 may be received by a reception apparatus via a plurality of transmission paths due to reflection by buildings and similar. The radio signal propagating on a plurality of different transmission paths undergo interference, and at the reception apparatus the reception strength of radio signal may fluctuate considerably. Such a phenomenon is sometimes called multipath fading. In a multipath fading radio communication environment, a particular subcarrier or frequency may be affected by multipath fading. FIG. 21 illustrates an example of the frequency spectrum of reception signal in a reception apparatus; the horizontal axis represents frequency and the vertical axis represents reception power. In FIG. 21, frequencies indicated by arrows have reduced reception power compared with elsewhere, and are affected by multipath fading.
On the other hand, in the case of the OFDM method, data patterns are mapped to each subcarrier in the frequency domain (see for example FIG. 19A), so that data is mapped over all subcarriers at system frequencies, and transmission power is substantially the same in system frequency bands. Hence in the case of the OFDM method, the transmission spectrum does not depend on the data pattern.
However, in the case of the SC-OFDM method, processing for conversion into the frequency domain by DFT processing is included (see for example FIG. 19B), and so in subcarriers after conversion there are cases in which subcarriers to which data is not mapped also exist. In such cases, the transmission power is not the same over all frequency bands, and the transmission spectrum depends on the data pattern. Hence in the case of the SC-OFDM method, depending on the preamble pattern, power peaks may occur in specific subcarriers, and there exist patterns for which power is unevenly distributed in specific subcarriers.
When such a subcarrier is for example affected by multipath fading, the correlation power characteristic of the preamble section is also affected. For example, in a subcarrier affected by multipath fading in the reception spectrum of FIG. 20, the correlation power characteristic of a preamble pattern such that the transmission spectrum power peaks or similar will be affected. In such cases, it may happen that for example the correlation power for the preamble section does not reach a presumed correlation peak power, or the correlation peak power may be detected with a timing other than the timing at which the correlation peak power is normally obtained. FIG. 20B illustrates an example of the correlation power in the preamble section in a case where the power is affected by multipath fading. In the case of FIG. 20B, the difference between the correlation peak power and the second correlation power is smaller than a threshold value, or is smaller than the case in which the correlation peak power is as in FIG. 20A. In such cases, the reception apparatus may not use the timing of the correlation peak power as reference, and reception synchronization may not be established based on the preamble pattern.
Further, in the above-described technique of sending a preamble employing two sub-patterns with phase inverted, only phase inversion is performed, and transmission employs the SC-OFDM method, so that in some cases power is unevenly distributed in particular subcarriers. Hence when using this technique as well, there are cases in which, because both of two sub-patterns are affected by multipath fading, the correlation characteristic of the preamble section is affected.
Further, even in the case of a technique for estimating an integer carrier frequency offset, a known preamble pattern is transmitted without modification, so that when transmission uses the SC-OFDM method, power is distributed unevenly to particular subcarriers and similar, and there are cases in which multipath fading has an effect.
Hence whatever the technique used, a particular subcarrier is affected by multipath fading in the preamble section, and there are cases in which reception synchronization cannot be established.